[flang][OpenMP] Add test: named COMMON + member with firstprivate+lastprivate is valid

Gate test to OpenMP 5.1+ and add explanatory comment per review feedback.

Author: Krish Gupta

flang/test/Semantics/OpenMP/omp-common-fp-lp.f90

@@ -0,0 +1,21 @@
+! RUN: %flang_fc1 -fopenmp -fopenmp-version=51 -Wno-experimental-option -fsyntax-only %s 2>&1 | FileCheck %s --allow-empty
+! CHECK-NOT: error:
+! CHECK-NOT: warning:
+
+! Regression test for issue #162033.
+! Verify that a named COMMON block can appear in a data-sharing clause together
+! with one of its members in another clause on the same construct. This is valid
+! in OpenMP >= 5.1 because:
+!  - A named COMMON in a clause is equivalent to listing all its explicit members
+!  - A list item may appear in both FIRSTPRIVATE and LASTPRIVATE on the same directive
+! OpenMP 5.0 explicitly forbade this combination.
+
+subroutine sub1()
+  common /com/ j
+  j = 10
+!$omp parallel do firstprivate(j) lastprivate(/com/)
+  do i = 1, 10
+     j = j + 1
+  end do
+!$omp end parallel do
+end